Though his appearance may confuse some, K'nuckles considers himself human. K'nuckles has a severe case of "whiskers," which causes his entire head to appear blue. The whiskers surrounding his mouth are ironically fake, of which he is greatly ashamed, as seen in "Beard Buddies." He also has scars and numerous tattoos, as evidenced when he is seen shirtless. He is 5'3".
K'nuckles is characterized by his long, orange nose, of which he is very fond of. He does not have any natural limbs left; they have all been replaced by crude, prosthetic wood, including the legs and hands, even his rear end is wooden, being a just a plank attached with a belt. Although he eventually reclaimed his real bottom, he lost it in the same episode, "Sittin' Muscle." K'nuckles has an odd waddle when he walks because of this and he makes squeaking noises whenever he moves.
K'nuckles lost one of his hands to a food processor at a young age, he once carved himself a new hand out of wood. Unfortunately, it was a mirror image of the one he needed; he then amputated his working hand and carved a second wooden hand. Fortunately, both prosthetic hands are in excellent working order, allowing K'nuckles to do anything a person with real hands can do (it has been said that he based them off the designs of the best-created mechanical hands ever created, although it is unknown if this is real or he just said this to boost his ego).
K'nuckles constantly wears a red pirate hat and jacket over light brown striped shirt underneath. His hat was given to him by a hatter who, when young K'nuckles asked him if he could have a hat, got one from the harbor ground with a bird nesting in it. Although the hat was covered with bird droppings, K'nuckles was still very proud of having it. He also has also been shown to wear bloomers, as "real men don't wear pants." as he has stated in "That's a Wrap!"
When his irises are seen, it is shown that he has light brown eyes. He also has black circles around his eyes. K'nuckles enjoys talking to birds. In the episode "Gone Wishin'," it has shown K'nuckles returning to his original stubble-less human form after making a wish from a mermaid's heart. It is unclear, however, if this is truly his actual normal form.
K'nuckles is not very active and prefers to spend his time sleeping in Bubbie's mouth or eating candy at the Candy Barrel (despite him and Flapjack not having enough money,). He usually likes to tell stories of his many adventures to Flapjack and, because of this, the young boy fails to see K'nuckles as the inactive good-for-nothing that he is. It is even heavily implied in many episodes that Flapjack may even be a better adventurer than K'nuckles himself due to the latter's own laziness and many lies told compared to Flapjack's own eagerness.
K'nuckles is not very bright, as shown in his attempt to do his laundry. Instead of going to the laundromat, he throws his clothes into the ocean and expects them to be thrown back into his arms once they're clean. He also shows some disgust towards emotional moments, but in the episode K'nuckles and His Hilarious Problem it is revealed that after he had too much candy he went into some sort of inebriated state where he confessed to Flapjack that he was the only friend he ever had and that he also cared for Bubbie. After his confession, he began to cry and soon passed out.
Though K'nuckles is shown to neglect Flapjack and their friendship on multiple occasions, he also shows some compassion toward Flapjack and is clearly fond of him as a friend. He might even look at Flapjack as the son he never had. K'nuckles seems to have an obsession with Maple Syrup which he also refers to as his friend. He is also illiterate, as seen in many episodes, particularly in "Candy Casanova" where he writes a letter to Candy Wife (which Flapjack reads) using only pictures; in Wishing Not So Well, he creates a to-do list using pictures as well.
Although many episodes draw attention to his inability to read, at the end of Revenge he handed Flapjack a letter that he seemingly wrote.
It is possible but unlikely that he dictated the letter, or that Flapjack somehow interpreted the season two.
Nobody knows what species he is in "Oh You Animal!", the lady for "Stormalong Second Chance for Boys" mistakes him for a strange animal, however in the episode Gone Wishin', it is revealed that K'nuckles was actually a human under some sort of curse as he stated to be wished back to normal.
K'nuckles appeared in the OK K.O. Let's Be Heroes episode, Crossover Nexus, as one of the Cartoon Network heroes that were turned into stone by Strike. He later appears as one of Ben 10's transformations to defeat Strike.